Bit line control circuit for semiconductor memory device

ABSTRACT

A semiconductor memory device includes a bit line sense amplifier for sensing and amplifying data applied on a bit line; a first driver for driving a pull-up voltage line of the bit line sense amplifier to a voltage applied on a normal driving voltage terminal; an overdriving signal generator for generating an overdriving signal defining an overdriving period in response to an active command; an overdriving control signal generator for receiving the overdriving signal to generate an overdriving control signal for selectively performing an overdriving operation according to a voltage level of an overdriving voltage; and a second driver for driving the normal driving voltage terminal to the overdriving voltage in response to the overdriving control signal.

FIELD OF THE INVENTION

The present invention relates to a semiconductor memory device; and,more particularly, to a bit line overdriving control circuit for use ina semiconductor memory device.

DESCRIPTION OF RELATED ART

As semiconductor memory chips are scaled down in line width and cellsize, a power supply voltage becomes lower. Accordingly, there is ademand for semiconductor memory devices that can satisfy the low voltagerequirement.

Most of the semiconductor memory chips include internal voltagegenerators that generate a plurality of internal voltages from anexternal voltage. Thus, the semiconductor memory chips supply internalcircuits with the internal voltages by themselves. In the memory devicessuch as DRAM using a bit line sense amplifier (BLSA), a core voltageVCORE corresponding to a voltage level of data “1” is used to detectcell data.

When a word line selected by a row address is activated, data of aplurality of memory cells connected to the word line are transferred tobit lines, and a bit line sense amplifiers sense and amplify voltagedifferences between bit line pairs. When thousands of bit line senseamplifiers operate at a time, a large amount of current is consumed at acore voltage (VCORE) terminal used to drive a pull-up voltage line ofthe bit line sense amplifiers. However, it is difficult to amplify alarge amount of cell data for a short time by using the core voltage(VCORE) in the low voltage environment.

To solve these problems, a BLSA overdriving method has been adoptedwhich drives the pull-up voltage line of the bit line sense amplifier ata voltage (generally, an external voltage (VDD)) higher than the corevoltage (VCORE) for a predetermined time in an initial operation of thebit line sense amplifier, that is, just after charge sharing between amemory cell and a bit line.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide asemiconductor memory device for preventing a voltage level of a VCOREterminal from rapidly increasing when a bit line sense amplifieroperates an overdriving operation in an environment of a relatively highexternal voltage.

In accordance with an aspect of the present invention, there is provideda semiconductor memory device including: a bit line sense amplifier forsensing and amplifying data applied on a bit line; a first driver fordriving a pull-up voltage line of the bit line sense amplifier to avoltage applied on a normal driving voltage terminal; a second driverfor driving the normal driving voltage terminal to an overdrivingvoltage; an overdriving signal generator for generating an overdrivingsignal defining an overdriving period in response to an active command;a level follower for outputting a linearly changing voltage with respectto the overdriving voltage; a voltage level detector for detectingwhether or not the overdriving voltage is higher than a predefined levelin response to the output voltage of the level follower; and a selectiveoutput unit for selectively outputting the overdriving signal inresponse to an output signal of the voltage level detector, wherein thesecond driver is controlled by the output signal of the selective outputunit.

In accordance with another aspect of the present invention, there isprovided a bit line sense amplifier control circuit including: a bitline sense amplifier for sensing and amplifying data applied on a bitline; a first driver for driving a pull-up voltage line of the bit linesense amplifier to a voltage applied on a normal driving voltageterminal; an overdriving signal generator for generating an overdrivingsignal defining an overdriving period in response to an active command;an overdriving control signal generator for receiving the overdrivingsignal to generate an overdriving control signal for selectivelyperforming an overdriving operation according to a voltage level of anoverdriving voltage; and a second driver for driving the normal drivingvoltage terminal to the overdriving voltage in response to theoverdriving control signal.

In accordance with further another aspect of the present invention,there is provided a bit line sense amplifier control circuit including:a bit line sense amplifier for sensing and amplifying data applied on abit line; an overdriving signal generator for generating an overdrivingsignal defining an overdriving period in response to an active command;an overdriving control signal generator for receiving the overdrivingsignal to generate an overdriving control signal for selectivelyperforming an overdriving operation according to a voltage level of anoverdriving voltage; and a driver for driving a pull-up voltage line ofthe bit line sense amplifier to the overdriving voltage in response tothe overdriving control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the preferredembodiments given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of a bit line sense amplifier controlcircuit in accordance with a first embodiment of the present invention;

FIGS. 2A to 2C are graphs illustrating changes of voltage level at acore voltage terminal when the bit line sense amplifier operates;

FIG. 3 is a circuit diagram of a bit line sense amplifier controlcircuit for selectively outputting an overdriving signal in accordancewith a second embodiment of the present invention;

FIG. 4 is a detailed circuit diagram of an overdriving control signalgenerating unit shown in FIG. 3; and

FIGS. 5A and 5B are timing diagrams of the semiconductor memory deviceillustrated in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

A bit line control circuit for a semiconductor memory device inaccordance with exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a BLSA control circuit in accordancewith a first embodiment of the present invention.

Referring to FIG. 1, the bit lines sense amplifier control circuit 100includes a pull-up voltage line RT0 and a pull-down voltage line SB,each coupled to a BLSA 120. Also, the bit line sense amplifier includesa plurality of driver transistors M1, M2 and M3 for driving the voltagelines RT0 and SB to a specific voltage.

Specifically, a second driver transistor M2 drives the pull-up voltageline RT0 to a voltage of the VCORE terminal in response to a pull-updriving control signal SAP, and a third driver transistor M3 drives thepull-down voltage line SB to a ground voltage VSS in response to apull-down driving control signal SAN. A first driver transistor M1drives the VCORE terminal to an external voltage VDD in response to anoverdriving signal OVDP.

An overdriving signal generator 140 generates the overdriving signalOVDP in response to an active command ACT. The first and second drivertransistors M1 and M2 may be implemented with PMOS transistors.

When the active command ACT is input, a word line WL is activated and acell data is loaded on a bit line pair BL and /BL by charge sharing.Then, the pull-up driving control signal SAP and the pull-down drivingcontrol signal SAN are activated to a logic high level. At this point,the pull-up voltage line RT0 is overdriven during a predeterminedduration by the overdriving signal OVDP that is activated to a logichigh level earlier than the pull-up driving control signal SAP and thepull-down driving control signal SAN. That is, when the pull-up drivingcontrol signal SAP, the pull-down driving control signal SAN, and theoverdriving signal OVDP are all activated to a logic high level, thedriver transistors M1, M2 and M3 are all turned on, so that the pull-upvoltage line RT0 and the pull-down voltage line SB are driven to theexternal voltage VDD and the ground voltage VSS, respectively.

After a predetermined time, the overdriving voltage OVDP is deactivatedto a logic low level so that the first driver transistor M1 is turnedoff. Therefore, the pull-up voltage line RT0 is driven only by the corevoltage VCORE.

FIGS. 2A to 2C are graphs illustrating changes of voltage level at theVCORE terminal when the bit line sense amplifier operates.

Specifically, FIG. 2A is a graph illustrating change of voltage level atthe VCORE terminal when the bit line sense amplifier does not perform abit line overdriving operation. After an active command ACT0 is applied,the voltage level of the VCORE terminal is rapidly lowered.

Currently, an external voltage VDD applied to the DRAM is 1.7-1.9 V. Tomeet the specification of 1.7-1.9 V, the DRAM must be able to normallyoperate in the external voltage (VDD) environment of less than 1.7 V ormore than 1.9 V.

FIG. 2B is a graph illustrating change of voltage level at the VCOREterminal when the bit line sense amplifier performs the bit lineoverdriving operation in an environment of a relatively low externalvoltage VDD. It can be seen from FIG. 2B that the VCORE terminalmaintains a stable level due to the overdriving operation.

FIG. 2C is a graph illustrating change of voltage level at the VCORElevel when the bit line sense amplifier operates the overdrivingoperation in an environment of a relatively high external voltage VDD.In this case, a voltage difference between the core voltage VCORE andthe external voltage VDD is large. Therefore, when the overdrivingoperation is performed in response to the active commands ACT0 and ACT1,the excessively high external voltage VDD is connected to the VCOREterminal and an amount of charges supplied to the VCORE terminal rapidlyincreases, causing a rapid increase of the core voltage level. Moreover,when the active commands ACT0 and ACT1 are consecutively applied, thecore voltage level increases much more due to remaining charges at theVCORE terminal.

In this case, the selected word line is driven by a boosted voltage(VPP) higher than the external voltage (VDD), and the bit line exhibitsa voltage level higher than the core voltage VCORE that is a normalvoltage level. Thus, a gate-source voltage (Vgs) of the cell transistoris reduced. If the gate-source voltage (Vgs) of the cell transistor isreduced, the read and write operations are not correctly carried out,causing the erroneous operations of the semiconductor memory device.

FIG. 3 is a circuit diagram of a BLSA control circuit 200 forselectively outputting an overdriving signal in accordance with a secondembodiment of the present invention.

As shown, the BLSA control circuit 200 includes a BLSA 220, anoverdriving signal generator 240, a plurality of driver transistors NM1,NM2 and NM3 and an overdriving control signal generator 400.

The overdriving signal generator 240 generates an overdriving signalOVDP in response to an active command ACT. The BLSA 220 is coupledbetween a pull-up voltage line RT0 and a pull-down voltage line SB. Theplurality of driver transistors NM1, NM2 and NM3 drives the voltagelines RT0 and SB to a specific voltage. The overdriving control signalgenerator 400 selectively outputs the overdriving signal OVDP accordingto supply voltage circumstance, i.e., whether the supply voltage is arelatively high voltage or a relatively low voltage.

Specifically, a second driver transistor NM2 drives the pull-up voltageline RT0 to a voltage of the VCORE terminal in response to a pull-updriving control signal SAP, and a third driver transistor NM3 drives thepull-down voltage line SB to a ground voltage VSS in response to apull-down driving control signal SAN. A first driver transistor NM1drives the VCORE terminal to a supply voltage VDD in response to anoverdriving signal OVDP. The first and second driver transistors NM1 andNM2 may be implemented with PMOS transistors.

When the active command ACT is input, a word line WL is activated and acell data is loaded on a bit line pair BL and/BL by charge sharing.Then, the pull-up driving control signal SAP and the pull-down drivingcontrol signal SAN are activated to a logic high level. At this point,the pull-up voltage line RT0 is overdriven during a predeterminedduration by the overdriving signal OVDP that is activated to a logichigh level earlier than the pull-up driving control signal SAP and thepull-down driving control signal SAN. That is, when the pull-up drivingcontrol signal SAP, the pull-down driving control signal SAN, and theoverdriving signal OVDP are all activated to a logic high level, thedriver transistors NM1, NM2 and NM3 are all turned on, so that thepull-up voltage line RT0 and the pull-down voltage line SB are driven tothe supply voltage VDD and the ground voltage VSS, respectively.

After a predetermined time, the overdriving voltage OVDP is deactivatedto a logic low level so that the first driver transistor NM1 is turnedoff. Therefore, the pull-up voltage line RT0 is driven only by the corevoltage VCORE.

The overdriving control signal generating unit 400 detects a voltagelevel of the source voltage VDD to output the overdriving signal OVDP asan output signal when the voltage level of the source voltage VDD islower than a predetermined voltage level and intercept the overdrivingsignal OVDP when the voltage level of the source voltage VDD is higherthan a predetermined voltage level. Accordingly, the BLSA controlcircuit 200 performs an overdriving operation in an environment of arelatively low external voltage and does not perform an overdrivingoperation in an environment of a relatively high external voltage.

FIG. 4 is a detailed circuit diagram of the overdriving control signalgenerating unit 400 shown in FIG. 3.

Referring to FIG. 4, the control signal generating unit 400 includes alevel follower 401, a voltage level detector 402 and a selective outputunit 403. The level follower 401 generates an output voltage A forlinearly changing with respect to a source voltage VDD. The voltagelevel detector 402 detects whether or not an overdriving voltage ishigher than a predefined level in response to the output voltage A ofthe level follower 401. The selective output unit 403 selectivelyoutputs an overdriving signal OVDP in response to an output signal B ofthe voltage level detector 402.

The level follower 401 includes first and second resistors R1 and R2connected in series between a source voltage (VDD) terminal and a groundvoltage (VSS) terminal to divide the source voltage VDD according to aresistance ratio of the first and second resistors R1 and R2 andgenerate the divided voltage as the output voltage A to a common node ofthe two resistors R1 and R2. For example, when the first and secondresistors R1 and R2 have the same resistance, the output voltage A ofthe level follower 401 is a half of the source voltage VDD, i.e., VDD/2.

The voltage level detector 402 may include an NMOS transistor NM4 havinga gate receiving the output signal A of the level follower 401 and beingconnected to the ground voltage (VSS) terminal, and a PMOS transistorPM1 having a gate receiving the ground voltage and being connected tothe source voltage (VDD) terminal.

The selective output unit 403 includes a NAND gate NAND receiving theoutput signal B of the voltage level detector 402 and the overdrivingsignal OVDP, and an inverter INV for inverting an output signal of theNAND gate NAND to output a signal C as an output signal of the selectiveoutput unit 403.

FIGS. 5A and 5B are timing diagrams of the semiconductor memory deviceillustrated in FIG. 4.

Specifically, FIG. 5A is a timing diagram when an overdriving to thesupply voltage VDD is required because a voltage difference between thesupply voltage VDD and the core voltage VCORE is not large. In FIG. 5A,reference symbols ‘AA’ and ‘BB’ represent the overdriving signal OVDPand the signal C of the selective output unit 403 of a BLSA controlcircuit 200 shown in FIG. 4.

The supply voltage VDD has about a voltage level of 1.6 V. Thus, it canbe seen that the voltage difference between the reference symbol ‘AA’and the core voltage VCORE of 1.5V is small.

The output voltage A of the level follower 401 is input to the voltagelevel detector 402. At this point, the NMOS transistor NM4 is not turnedon by its threshold voltage so that the output voltage B of the voltagelevel detector 402 becomes a logic high level. Therefore, the selectiveoutput unit 403 outputs the overdriving signal OVDP as the final outputsignal C. Because the normal bit line overdriving operation is carriedout and the supply voltage VDD is relatively low, the core voltage VCOREcan be stably maintained even if the bit line overdriving operation iscarried out.

FIG. 5B is a timing diagram when an overdriving to the supply voltageVDD is not required because a voltage level difference between thesupply voltage VDD and the core voltage VCORE is large.

In FIG. 5B, reference symbols ‘CC’ and ‘DD’ represent the overdrivingsignal OVDP and the signal C of the selective output unit 403 of a BLSAcontrol circuit 200 shown in FIG. 4.

The supply voltage VDD has about a voltage level of 2.2 V. Thus, it canbe seen that the voltage difference between the sense amplifieroverdriving signal CC and the core voltage of 1.5V is large.

The output voltage A of the level follower 401 is input to the voltagelevel detector 402. At this point, the NMOS transistor NM4 is turned onso that the output voltage B of the voltage level detector 402 becomes alogic low level. Therefore, the selective output unit 403 does notoutput the overdriving signal OVDP, so that the final output signal C isdeactivated to a logic low level. In this case, the bit line overdrivingoperation is omitted and the normal driving operation is carried out.Consequently, the excessive increase of the cover voltage level can beprevented while the overdriving operation is carried out in anenvironment of a relatively high supply voltage VDD.

The kinds and arrangement of the logics used in the above-describedembodiments have been implemented for the case where both the inputsignal and the output signal are the active high signals. Thus, if theactive polarity of the signals is changed, the logic configurations willalso be changed. In addition, these logic configurations can be easilyderived by those skilled in the art.

The resistors of the logic followers may be replaced with activeelements such as PMOS transistor and NMOS transistor.

Although the selective output unit has been implemented using the ANDgate that performs AND operation of the detected signal and theoverdriving signal, it can also be implemented using various ways. Forexample, the overdriving signal can be selectively output using atransmission gate and a latch controlled by the detected signal.

In addition, although the core voltage and the external voltage havebeen used as the normal driving voltage and the overdriving voltage, thepresent invention is not limited to these voltages.

As described above, the present invention can prevent the voltage levelof the core voltage from excessively increasing due to the overdrivingoperation when the bit line sense amplifier operates in the environmentof the relatively high external voltage, thereby improving the operationcharacteristic and reliability of the semiconductor memory device.

The present application contains subject matter related to Korean patentapplication No. 2005-90911 & 2005-132504, filed in the KoreanIntellectual Property Office on Sep. 29, 2005 & Dec. 28, 2005, theentire contents of which is incorporated herein by reference.

While the present invention has been described with respect to certainpreferred embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

1. A semiconductor memory device, comprising: a bit line sense amplifierfor sensing and amplifying data applied on a bit line; a first driverfor driving a pull-up voltage line of the bit line sense amplifier to avoltage applied on a normal driving voltage terminal; a second driverfor driving the normal driving voltage terminal to an overdrivingvoltage; an overdriving signal generator for generating an overdrivingsignal defining an overdriving period in response to an active command;a level follower for outputting a linearly changing voltage with respectto the overdriving voltage; a voltage level detector for detectingwhether or not the overdriving voltage is higher than a predefined levelin response to the output voltage of the level follower; and a selectiveoutput unit for selectively outputting the overdriving signal inresponse to an output signal of the voltage level detector, wherein thesecond driver is controlled by the output signal of the selective outputunit. 2-19. (canceled)